Water treatment processes commonly utilize microbes, such as bacteria, to catalyze the degradation of unwanted materials in water. The microbes use the unwanted material as a fuel source thereby removing it from the water. Some standard processes for biological purification of wastewater include activated sludge, trickling filter, and rotary disk aeration processes and the like.
One problem common to these conventional water treatment processes is that they require large equipment and process footprints because of their small treatment capacity per unit volume. The activated sludge process suffers from other particular deficiencies. For example, it requires aeration of wastewater through the intensive introduction of air bubbles to the water (“sparging”), which can be very expensive due to the large amounts of energy needed to operate conventional sparging equipment, and which causes difficulty in controlling aeration and mixing of the wastewater independently. In addition, sparging is inefficient in that a large percentage of the input gas is lost when the bubbles burst at the top of the aeration tank, unless capital-intensive gas recycling is used. Another disadvantage of the activated sludge process is that the population of microbes in the reactor typically comprises mainly aerobic microbes, whereas anaerobic microbes are additionally required for the complete or nearly complete removal of some contaminants. Yet another disadvantage is that the process requires the treated liquid, including the microbes in the liquid, to go to settling tanks where it remains for some time to allow the microbes to settle out of the liquid, so that they can be recycled back to the reactor. Still another disadvantage of the activated sludge process is that it generates a large quantity of excess microbes, the disposal of which is energy intensive and costly.
A variety of membrane technologies have been developed to address some of the problems presented by the treatment of wastewater. For example, membrane bioreactors (MBRs) have been developed to replace the conventional secondary settling tanks commonly found in municipal wastewater treatment plants. In this capacity, MBRs essentially function as liquid filters. Water in the activated sludge tank is drawn through the water-permeable MBR membrane, while suspended solids, bacteria, and most viruses are retained. By acting as a filter for bacteria, MBRs provide a number of advantages over more conventional settling tanks, such as providing higher microbial cell retention times and concentrations, reduced production of excess microbes, and smaller process footprints.
In addition, membrane aeration bioreactors (MABRs) have been developed with the potential to replace the conventional air spargers commonly used in activated sludge tanks. These spargers serve to deliver air bubbles (containing oxygen) to suspended bacteria, which catalyze the oxidation of organic contaminants in the water. Using a MABR, a microbial film is grown on a water impermeable, gas permeable membrane, and a gas is delivered directly to the microbial film through the membrane. When air or another oxygen-containing gas is supplied through the membranes, the resulting microbial film may comprise both aerobic and anaerobic types of bacteria in a wider variety than that typically found in a conventional sparged tank, thereby resulting in enhanced removal of nitrogen and other contaminants. MABRs are also a more energy efficient means for the delivery of oxygen to the microbes and thus are potentially less expensive to operate than conventional air spargers. Besides oxygen-containing gases, MABR membranes may be used to deliver other gases to microbes in water. For example, gas mixtures containing methane may be advantageously used when the microbial population comprises methylotrophic bacteria.
Extractive membrane bioreactors (EMBRs) are a third membrane-based technology used in wastewater and waste gas treatment. In this application, membranes are used to extract degradable, water-soluble organic molecules from a fluid into an aqueous medium. The extracted organic constituents are treated by microorganisms disposed either in the aqueous medium or in an external biological reactor.
Other applications of membrane technologies include liquid degassing, in which a soluble gas is extracted from a liquid (e.g., water) disposed on one side of a gas permeable membrane by crossing the membrane into a liquid or gas disposed on the opposite side of the membrane. Liquid degassing is useful, for example, in the production of ultrapure water. A similar process, called pervaporation, is used to extract volatile organic compounds from a liquid disposed on one side of a selectively permeable membrane, the volatile organic compounds passing through the membrane into a fluid stream disposed on the opposite side. Membrane technologies are also used for humidification, in which an initially dry gas disposed on one side of a selectively permeable membrane becomes humidified by the passage of water vapor across the membrane, the water vapor originating from liquid water disposed on the opposite side of the membrane. Another application of membrane technologies is liquid gasification, in which at least one constituent of a gas mixture disposed on one side of a selectively permeable membrane is transported through the membrane and thereby dissolved in a liquid disposed on the opposite side of the membrane.
The fluid membrane devices described above, e.g. MBRs, MABRs and/or EMBRs, generally have one of the following membrane constructions: tubular, hollow fiber, or flat sheet porous membranes. Flat sheet porous membranes can be assembled into pleated cartridges, spirally-wound modules, or plate-and-frame configurations. Plate-and-frame flat sheet membrane modules are typically easier to clean than other types of membrane modules. Flat sheet porous membranes that are included as part of plate-and-frame modules along with hollow fibers membranes are the predominate forms of membrane configurations currently used in the processing of wastewater. However, need remains for membrane devices that are capable of assisting in water and wastewater treatment, especially on the larger scale required for municipal and/or industrial wastewater treatment.